A knee joint is a joint that is constructed with a femur, a tibia, and a kneepan. A knee cartilage or a meniscus is located between a distal end of the femur and a proximal end of the tibia, and acts as a cushion in the knee joint, whereby the knee joint can smoothly work.
However, when the knee cartilage or the meniscus is worn off due to fatness or aging, not only a cushioning property between the distal end of the femur and the proximal end of the tibia is lost, but also the knee joint is possibly deformed. The knee joint is deformed when joint rheumatism is gotten, or when the knee is injured. For the deformation (knee osteoarthritis) of the knee joint, the knee joint cannot smoothly work, but a patient feels a bad pain in walking, and sometimes the patient has difficulty walking.
A total knee replacement is adopted as a treatment technique for the knee osteoarthritis. In the total knee replacement, the distal end of the femur and the proximal end of the tibia are cut off, and the cut portion is replaced with the artificial knee joint. Currently many patients are subjected to the total knee replacement, which allows relief of the pain or performance of the normal walking. Therefore, many patients highly satisfy the total knee replacement. There have been developed many artificial knee joints used in the total knee replacement (see Patent Documents 1 and 2).
In the knee joint, the femur and the tibia are joined to each other by a ligament in order to stabilize the working and attitude of the knee joint. However, sometimes the ligament joined to the distal end of the femur and the proximal end of the tibia, namely, an anterior cruciate ligament or a posterior cruciate ligament is cut off because the distal end of the femur and the proximal end of the tibia are cut off in performing the total knee replacement. Currently there are two methods, namely, a method (see Patent Document 1) in which both the cruciate ligaments are cut off to substitute a ligament function for the artificial knee joint and a method (see Patent Document 2) in which the anterior cruciate ligament is cut off while the posterior cruciate ligament is preserved. The method to be adopted is selected according to a damaged condition of the joint or ligament.
The anterior cruciate ligament is removed in both the methods. Although the artificial knee joint can be substituted for the function of the anterior cruciate ligament, the artificial knee joint is far inferior to the function of the original anterior cruciate ligament. Therefore, a patient having the artificial knee joint does not feel inconvenience in normal walking too much, but feels inconvenience when going up and down the stairs.
A patient having the artificial knee joint due to an injury of sports has a strong demand to do sports even if having the artificial knee joint. However the above methods cannot respond to the demand.
An artificial knee joint having a form of preserving the anterior cruciate ligament is also developed in order to respond to the demand (Patent Document 3). However, the anterior cruciate ligament is frequently damaged in the knee osteoarthritis, and sometimes a ligament length changes in association with the joint deformation. In the artificial knee joint of Patent Document 3, even if the anterior cruciate ligament is preserved, the anterior cruciate ligament hardly exerts the sufficient function. In the knee osteoarthritis caused by the damaged anterior cruciate ligament, even if the anterior cruciate ligament is preserved, the function of the anterior cruciate ligament can hardly be expected to be exerted.
Patent Document 4 discloses a technique of joining a femur member and a tibia member to each other by an artificial ligament as a mechanism preventing mobilization of an insert in an artificial knee joint including the tibia member and an insert mobile mechanism (a mechanism in which the insert moves on the tibia member with a freedom degree). In the technique of Patent Document 4, an end of a tibia-side member of the artificial ligament is joined to tibia-side member with an elastic member such as a spring interposed therebetween, which allows hardness of the artificial ligament to be brought close to natural hardness.